1. Field of the Invention
This invention relates generally to optical detection devices, and, in particular, to a device and method for performing spectral measurements in flow cells with spatial resolution.
2. Description of the Related Art
Microfluidic devices have recently become popular for performing analytical testing. Using tools developed by the semiconductor industry to miniaturize electronics, it has become possible to fabricate intricate fluid systems which can be inexpensively mass produced. Systems have been developed to perform a variety of analytical techniques for the acquisition of information for the medical field.
A process called "field-flow fractionation" (FFF) has been developed to separate and analyze micromolecules and particles for analysis by the use of a force applied across a flow channel carrying a variety of particle sizes. Examples of this method are taught in U.S. Pat. Nos. 3,449,938; 4,147,621; 4,214,981; 4,830,756; and 5,156,039.
A related method for particle fractionation is the "Split Flow Thin Cell" (SPLITT) process. this process has been used to develop devices having mesoscale functional element capable of rapid, automated analyses of preselected molecular or cellular analytes in a range of biological and other applications. Examples of this method are taught in U.S. Pat. Nos. 5,296,375; 5,304,487; 5,486,335; and 5,498,392.
Still another method used for assaying fluids involves application of electrical fields to a microfluidic system for providing capillary electrophoresis to separate materials in a flow channel. Examples of this process are taught in U.S. Pat. Nos. 5,699,157; 5,779,868; and 5,800,690.
U.S. Pat. No. 5,716,852 teaches yet another method for analyzing the presence and concentration of small particles in a flow cell using diffusion principles. This patent, the disclosure of which is incorporated herein by reference, discloses a channel cell system for detecting the presence of analyte particles in a sample stream using a laminar flow channel having at least two inlet means which provide an indicator stream and a sample stream, where the laminar flow channel has a depth sufficiently small to allow laminar flow of the streams and length sufficient to allow diffusion of particles of the analyte into the indicator stream to form a detection area, and having an outlet out of the channel to form a single mixed stream. This device, which is known as a T-Sensor, contains an external detecting means for detecting changes in the indicator stream. This detecting means may be provided by any means known in the art, including optical means such as optical spectroscopy, or absorption spectroscopy or fluorescence.
In a paper entitled "An Argument for a Filter Array vs. Linear Variable Filter in Precision Analytical Instrument Applications", the author discusses the advantages and disadvantages of several different types of optical filtering devices which can be employed in conjunction with a detector to enhance the effectiveness of the analytic equipment. The first type of filter described is a filter array (FA) which is composed of discrete segments each having a different bandpass and a uniform passband across each segment. This array is formed by cutting a finished filter into strips and assembling them into an array.
Another type of filter described is a linear variable filter (LVF). This filter is constructed by varying the thickness of the thin films which define the spectral characteristics. The wavelength changes as a function of thickness, creating a continuously variable passband along the length of the filter, with every segment, no matter how small, having a different passband.
One way to look at these filters is to think of the LVF as an analog device and the FA as a digital device, with each having certain advantages and disadvantages. For example, the LVF has the advantage that, regardless of the size and number of pixels behind the filter, each sees a different segment of the spectrum. The number of channels is limited only by the number of pixels and available energy. However, as the width of one segment of the LVF increases, the resolution decreases because each portion of the segment has a different passband. In addition, because it is the change in layer thickness of the given materials which determines the passband variance, the spectral region over which a single LVF can perform is limited by the properties of a set of coating materials, and the change in passband characteristics is determined by the coating design.
Advantages of the FA include: the spectral region can be very broad, as each segment can be made with different coating materials, making it possible to take advantage of the absorption characteristics of materials to achieve a very high rejection outside the passband; any segment can have spatial and optical characteristics totally independent of the other segments of the filter, and the elements can also be made in different widths, thus allowing wider segments in regions of lower sensitivity.
It would be desirable, particularly in the field of microfluidic flow analysis, to produce a filter which would allow variable transmission in any direction or geometry (e.g., a two-dimensional matrix which is variable in one dimension, and having variable optical density in the other dimension for the low-cost spectral analysis of a sample over an extremely wide dynamic range). This could be accomplished by a filter upon which, by microlithographic or printing techniques, various absorbing material is deposited or removed to form the desired absorption pattern. In its simplest form, the technique would involve loading an ink-jet type printer with an assortment of well-defined optically absorbing dyes, and printing the desired structures on a sheet of transparent material. Such a technique would permit the production of filters with variable transmission in any orientation and geometry.
The aforementioned T-Sensor device allows the fluorescence and absorption detection of analytes in complex samples based on diffusion separation in layers of laminar flows. By imaging an area of the T-Sensor, the fluorescence or absorption of so-called diffusion interaction zones between a sample, a detection, and a reference stream can be determined. The intensity of these diffusion interaction zones is then used to determine the analyte concentration. By placing a linear variable filter or a filter array in the optical path such that the transmission variation of the filter occurs in the flow direction, it is possible to spectroscopically determine absorption or fluorescence in a T-Sensor. A detecting means, such as a charge-coupled display (CCD) device, can then be positioned such that it will see slices of very similar cross sections of the T-Sensor channel, each measured at a different wavelength. Many analytic parameters can be derived from these cross section profiles, including reference background intensity and profile shape; reference interaction zone intensity, width, and shape of both dye and reference diffusion profile; detection solution background intensity and profile shape; sample interaction zone intensity, width, shape, of both dye and same diffusion profile; sample background intensity and profile shape; x-location of reference interaction maximum intensity; and x-location of sample interaction maximum intensity.